Chapter 4: Fuel Cells

How does a PEM fuel cell work?

Fuel (Hydrogen gas) is first transported to the anode of the cell. Then H2 undergoes the anode reaction. The anode reaction splits the fuel into H+ (a proton) and e-. The protons pass through the electrolyte to the cathode. Electrons cannot pass through the electrolyte and must travel through an external wire which becomes usable current. Protons and electrons reach the cathode, and undergo the cathode reaction.

How do fuel cell vehicles work, in a general sense?

Fuel cell cars and trucks combine hydrogen and oxygen to produce electricity and run a motor

What are the advantages of fuel cell technology?

It is a renewable energy source, it produces zero emissions (the only byproduct is water), it has high efficiency (no combustion involved), it operates quietly.

What is the structure of a PEMFC?

The basic physical structure consists of two electrodes with a conducting electrolyte membrane between them.

Talk about PEMFCs

The electrolyte in this fuel cell is an ion exchange membrane (such as Nafion, which is an excellent proton conductor). The only liquid in this fuel cell is water. Water management in the membrane is critical for efficient performance. It is useful for transportation because it works at low temperature.

What are the electrode materials in a FC?

gas diffusion layers (porous carbon paper, cloth), catalyst ink, electrode fabrication (ink deposition on carbon paper)

What is the theoretical open circuit voltage of a single fuel cell operating below 100C?

1.23 V

What are the operating temperatures of the common fuel cell types?

80C for PEMFC, 100C for AFC, 200C for PAFC, 650C for MCFC, 600-1000C for SOFC

What is the difference between a fuel cell and a battery?

A battery is an energy storage device. A battery will produce electrical energy when the chemical reactants are consumed. In a secondary battery, the reactants are regenerated by recharging, which involves putting energy into the battery from an external source. A fuel cell is an energy conversion device which can produce electrical energy as long as fuel and oxidant are supplied to the electrodes. Fuel cells operate like batteries, but do not run down or need recharging. They produce electricity and heat as long as fuel is supplied.

What is the polarization curve?

A common method for testing a fuel cell; displays the voltage output of fuel cell for a given current density loading

What is the basic principle of a fuel cell?

A fuel cell combines fuel (such as hydrogen) and oxidant (such as oxygen) electrochemically to produce electricity

What causes overpotential losses?

Activation loss, Ohmic loss (voltage drop due to resistance of the cell components and interconnections), mass transfer. Voltage decreases as current increases.

What are the three main reasons for voltage drop in fuel cells?

Activation polarization (charge transfer activation), ohmic loss (ion and electron transport), mass transport limitation

What are the four main components of fuel cells?

Anode, cathode, electrolyte (ion conductive material), catalyst (to improve the kinetics of electrochemical reactions)

What is the real cell voltage?

Around 0.6-0.7 V. The voltage loss between theoretical and real is known as overpotential.

Compare hydrogen and battery cars.

Both have zero emissions at the point of use. Hydrogen cars densely pack their energy storage and usually have a longer range (300 miles) than battery cars (100-200). Hydrogen cars are much more expensive ($20k more) than battery cars.

What is the major disadvantage of fuel cells?

COST

True or false: exothermic reactions absorb energy (+ enthalpy) while endothermic reactions release energy (- enthalpy).

False. Endothermic = absorb energy (+), exothermic = release energy (-)

True or False: Fuel cells have lower energy density than batteries.

False. Fuel cells have higher energy density.

What is MEA?

Membrane Electrode Assembly. MEA consists of gas diffusion layer/catalyst/membrane/catalyst/diffusion layer, all pressed together.

Why is fuel cell technology cost down 95%?

Platinum (very expensive) is being replaced as a catalyst material in fuel cells.

What are commonly used fuel cells?

Polymer Electrolyte Membrane (PEM; also called Proton Exchange Membrane), Alkaline Fuel Cell (AFC; use KOH as electrolyte), Phosphoric Acid Fuel Cell (PAFC; use liquid phosphoric acid as electrolyte; operates at 150-220 degrees C), Molten Carbonate Fuel Cell (MCFC), Solid Oxide Fuel Cell (SOFC)

True or False: Fuel cells have lower power density than batteries.

True

True or False: Operating temperature plays an important role in dictating the type of fuel that can be used in a fuel cell.

True

True or False: The MEA makes up about half the cost of an FC.

True, because of the platinum catalyst.

How do you charge a fuel cell?

You do not charge a fuel cell like you would a battery. Instead, you refuel it with fuel and oxidants that create electrochemical reactions. Therefore, refueling an empty reactant tank is much faster than recharging a battery.

What is a fuel cell?

electrochemical energy conversion devices that convert the chemical energy of a reaction directly into electrical energy

What are some applications of fuel cells?

portable electronics, automotives, distributed generation, military and aerospace, auxiliary power, back-up power

What is enthalpy?

the measure of how much energy is released or absorbed during a chemical reaction